Add Case-Aware Substitution With vim-abolish as a vim til.

vim/case-aware-substitution-with-vim-abolish.md

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+# Case-Aware Substitution With vim-abolish
+
+Substitution in vim is, by default, case-sensitive. Adding the `i` `s-flag`
+makes it case-insensitive.
+[`vim-abolish`](https://github.com/tpope/vim-abolish), on the other hand, lets
+you perform a case-insensitive substitution that preserves three case
+variants (foo, Foo, and FOO). Substitution with `vim-abolish` can be
+performed with `Subvert` or `S`.
+
+For instance, `:%S/blog/article/g` will turn
+
+```
+blog Blog bLOg BLOG
+```
+
+into
+
+```
+article Article bLOg ARTICLE
+```
+
+Install `vim-abolish` and see `:h Subvert` for more details.